Since the discovery of high temperature superconducting bismuth-based oxides in the late 1980's, there has been a frenzy of development aimed at manufacturing long lengths of wires and tapes for use in practical superconducting magnets. Currently, the ceramic powder in silver tube process, or more commonly known as Powder-In-Tube (PIT) method, is the only process capable of fabricating long length (more than 10 meters), mechanically robust High Temperature Superconductors (HTS) with good superconducting properties. In the PIT method, superconducting ceramic powders are loaded into a silver tube which is then mechanically worked utilizing extrusion, drawing, swaging, and rolling to produce long wires or tapes. A final heat treatment produces the superconducting phase. The PIT process has been, and continues to be, developed by trial and error, a time-consuming and expensive process. There is an urgent need for improved process quality management and control of PIT HTS manufacturing which would minimize the trial and error.
The development of advanced sensing techniques for process quality control throughout the various stages of HTS wire manufacturing promises to reduce manufacturing costs and enhance HTS wire quality. Uniformity of conductor dimensions and ceramic core density are of critical importance to the in-service performance of HTS wire. All current applications of HTS wire are basically static in nature, such as the large magnets in Magnetic Resonance Imaging (MRI) systems. The consequences of failure in these applications are relatively low. In the next generation of HTS wire implementation, the emphasis will be on dynamic, high stress applications such as transportation, power generation and storage where the consequences of failure are substantial. Process quality management will become a prerequisite to HTS wire implementation in these critical applications.
The silver sheath of silver sheathed ceramic powder core HTS wire significantly affects the mechanical properties of the HTS wire in several ways. First, silver sheathed tapes (obtained by rolling the wire) have proven to exhibit the highest critical current density (Jc) among all HTS products except for thin films. Second, the silver sheath is substantially inert and allows the penetration of oxygen during the heat treatment process and does not "poison" the superconductor. Third, the silver sheath provides mechanical strength and integrity to the wire. Its ductility helps in winding the wire to produce useful magnets (otherwise impossible to make out of a brittle ceramic material).
Given the importance of the silver sheath, its dimensional uniformity (implying a uniform ceramic core) is critical to the electrical performance characteristics of the HTS wire. For example, the non-uniformity of the ceramic core often results in a critical defect called sausaging. Sausaging defects typically occur during the last stages of the PIT process and results in a significant decrease in critical current density (Jc).
Consequently, the measurement of silver sheath thickness is important from both process development and in-process monitoring standpoints. Current quality control practices to measure silver sheath thickness in HTS wire employ destructive metallographic characterization techniques. These techniques involve careful sectioning, mounting and evaluation under an optical microscope. These techniques are destructive, inherently time consuming and cannot be used on routine evaluation on production wire. In this context, a sensor for continuous measurement of silver sheath thickness during wire fabrication offers considerable advantages. Presently, such sensors to measure sheath thickness in composite HITS wire do not exist. The development of a nondestructive technique and an associated sensor for real-time measurement during HTS wire drawing promises significant benefits.
Ultrasonic techniques have been shown capable of thickness measurements in a variety of applications. In this technique, the time of flight of an ultrasonic pulse in a material is used to determine thickness, assuming that the velocity of the pulse in the material is known. Ultrasonic techniques are non destructive, rapid and can be automated for real-time, continuous monitoring of HTS wire manufacturing. However, a sensor to measure silver sheath thickness in HTS wire based on such ultrasonic techniques has not been previously developed.